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Chapter 36. Management of the Ventilated Patient

Effective preventive measures in ventilated patients include raising the head of the bed during enteral feeding, using measures to prevent venous thromboembolism, avoiding unnecessary changes of the ventilator circuit, and reducing the amount of sedation.

Ventilator parameters should be determined by the pathophysiology underlying the particular form of respiratory failure; this approach facilitates stabilization and comfort of the patient on the ventilator, prevention of common complications, and early liberation from this supportive therapy.

Although some patients may require sedation and muscle relaxation for initial stabilization on the ventilator, these agents should not be used to routinely adapt the patient to the machine; rather, ventilator adjustments should be used to stabilize and comfort the patient. This latter approach is facilitated by a careful analysis of airway pressure and flow waveforms.

Whenever the adequacy of oxygen exchange is in question, the initial fraction of inspired oxygen (FiO2) should be 1.0; this will be diagnostic and therapeutic because failure to achieve full arterial hemoglobin saturation identifies a significant right-to-left shunt.

Typical ventilator settings for the patient with normal lung mechanics and gas exchange include an FiO2 of 0.5, tidal volume of 8 to 12 mL/kg, and respiratory rate of 8 to 12 breaths/min; if mechanical ventilation has been instituted to rest fatigued respiratory muscles, deep sedation may be necessary to minimize respiratory muscle activity.

The patient with severe airflow obstruction often develops hypoperfusion after institution of positive-pressure ventilation as a result of occult positive end-expiratory pressure (autoPEEP); this responds to temporary cessation of ventilation and vigorous volume resuscitation while measures are used to reduce airflow obstruction.

The goals of ventilator management in severe airflow obstruction include a plateau airway pressure below 30 cm H2O, an autoPEEP below 10 cm H2O, or an end-inspired lung volume smaller than 20 mL/kg even if this results in hypercapnia; short expiratory times must be avoided.

The patient with acute hypoxemic respiratory failure resulting from pulmonary edema benefits from lung-protective ventilation (6 mL/kg ideal body weight and rate approximately 30 breaths/min). The initial FiO2 of 1.0 can be lowered to nontoxic levels by raising PEEP, which is guided by pulse oximetry.

Too often, the management of the patient on a ventilator is guided by (a) a standard protocol applied to diverse patients regardless of the underlying pulmonary derangement or (b) mode-dominated thinking on the part of the physician, by which various microprocessor-controlled machine functions are hoped to have a salutary effect on patient outcome. This chapter offers an alternative approach, in which ventilator parameters are tailored to the patient's mechanical and gas exchange abnormalities. This facilitates early stabilization of the patient on the ventilator in such a way as to optimize carbon dioxide removal and oxygen delivery within the limits of abnormal neuromuscular function, lung mechanics, and gas exchange and limit complications of barotrauma, lung injury, and cardiovascular depression.